There's an element of truth to this. Iroquoian warfare often called for the capture of enemy populations as a primary goal. This was especially true during the Beaver Wars (1629 - 1701) when the Iroquoian war machine was at its peak. I go into much more detail about the fate of such captives in this post, but I'll do a quick summary here.

Immediately after being captured, the captive may be purposefully injured to make escape more difficult. This is more likely to be the case if the captive proved to be unruly or had made previous escape attempts. Officially the capturing warrior is tasked with bringing the captive back alive and has no authority to decide their fate.

Once returning home, the captives were divided up among the participating clans and were forced to run through a gauntlet, during which they could receive severe beatings. Then the women of the clans passed a final judgment on the captives. The worst outcome, execution by communal torture, was usually reserved only for enemy combatants or fighting-age men, more generally. It was a brutal and bloody affair, but sometimes even adult men would be spared this treatment. That became increasingly common as history went on, with European captives at least, because the clans realized Europeans would happily pay a ransom for the return of captives.

Normally women and children would end up being adopted into the clan that claimed them. At this point, they were effectively low-ranked members of the nations undergoing assimilation. Early on, this can involve having a lot of the family's menial labor dumped on you - hence the accusation of slavery - but over time, your position in the family normalized and you'd be able to participate in Iroquoian society like anyone else, barring a few exceptions. If you were a man, for example, you might not be allowed to participate in attacks against your original nation.